It isn't going to last for a long time, Portal has a novelty value that wears off once you know how to solve the levels, for me it wore out at about 30 hours. It was a great 30 hours though, it's definitely worth it.

If you want something that will last a long time, get something multiplayer.

It all depends what types of games you're interested in, but the most time consuming games tend to be those with large multiplayer influence, such as one of the many DotA games (HoN, LoL, DotA2) or an FPS.